The development of robust chemical sensors for a range of analytes remains an important endeavor for applications such as environmental monitoring, product quality control, and chemical dosimetry. Among the many methods available for chemical sensing, colorimetric techniques remain advantageous in that the human eye can be used for signal transduction, rather than extensive instrumentation.
Though colorimetric sensors currently exist for a range of analytes, most are based upon employing dyes or colored chemical indicators for detection. Such compounds are typically selective, meaning arrays are necessary to enable detection of various classes of compounds. Moreover, many of these systems have lifetime limitation issues, due to photo-bleaching or undesirable side reactions. Other optical sensing techniques, such as surface plasmon resonance and spectral interferometry, require substantial signal transduction hardware to provide response, and thus are not useful for simple visual indication.
Sensors are commonly used to detect the presence or absence of a particular chemical. Known sensors have been provided in a variety of forms for achieving this purpose. For example, signs and badges have been developed—see, for example, U.S. Pat. No. 6,284,198 and U.S. Patent Application 2004/0223876A1—along with filters, filter cartridges, and breathing masks (respirators) that include chemical indicators—see, for example, U.S. Pat. Nos. 5,323,774, 5,297,544, and 4,684,308. One particular chemical sensor that has been developed is a passive end-of-service-life indicator (ESLI) that has a film-like body. Within the film body is a detection layer that detects the presence of a particular chemical. Examples of film-like passive ESLIs are described in U.S. Patent Publications 2008/0063575A1 and 2008/0063874A1 to Rakow et al. ESLIs may be tailored to respond to various organic vapors and reactive gases.
ESLIs have been used in filter cartridges to assist in informing the user of when the cartridge has met the end of its service life—see, for example, U.S. Pat. Nos. 7,442,237, 6,497,756, 5,323,774, 5,297,544, 4,684,380, 4,530,706, 4,365,627, 4,326,514, 8,336,543, 8,225,782, 8,365,723, 7,503,962 and 4,154,586. The ESLIs are disposed adjacent to the housing sidewall so that they can be easily viewed from the exterior. The ESLIs are generally designed to be colorimetric sensors that is they change color after being exposed to a sufficient amount of contaminant in the filtered air. The color change and sensor placement are tailored to provide indication that coincides with when the filter cartridge service life ends. One particular problem with colorimetric ESLIs is that the wearer needs to observe the ESLI normal to the plane of the device to avoid reduction in contrast. With a reduction is contrast between exposed and unexposed areas of the ESLI, incorrect reading may be obtained.